The present invention relates to a process for preparing polyether polyols by base-catalyzed addition of alkylene oxides (epoxides) onto starter compounds which are solid at room temperature and have Zerevitinov-active hydrogen atoms.
Polyether polyols based on high-functionality starter compounds solid at room temperature, for example sugars, oligo- and polysaccharides, sugar alcohols (for example mannitol or sorbitol) and pentaerythritol, are generally reacted with polyfunctional isocyanates to give polyurethanes. It is equally possible, through the use of such polyether polyols, to obtain formed or solid polyurethane materials. Specific demands on the mechanical properties of the material, flammability, through-curing characteristics of the reaction components or hydrophilicity or hydrophobicity of the material are generally addressed via the structure of the polyether polyol and therein, in turn, through the choice of starter compound(s) and the composition of the added alkylene oxides. The alkylene oxides can be metered in a mixture or individually in succession, i.e. blockwise. Particular emphasis should be given here to the use of ethylene oxide as a pure block or to the use of blocks with a high ethylene oxide content as well as those formed from higher alkylene oxides, for example propylene oxide, since it is possible thereby to obtain not just polyurethane materials with elevated hydrophilicity but also, if the ethylene oxide has been metered in as an end block, polyether polyols having an elevated proportion of primary end groups, which impart to the polyether polyol elevated reactivity to isocyanates.
Starter compounds solid at room temperature can easily be made amenable to alkylene oxide addition reactions by performing the alkylene oxide addition in the presence of solvents unreactive towards alkylene oxides, as described, for example, in U.S. Pat. No. 4,332,936. In general, however, for reasons of sustainability and product hygiene, the use of organic solvents is undesirable. In addition, it is necessary in this case to provide valuable reactor volume for the solvent.
It is also possible to use starter compounds liquid at room temperature (co-starters) and/or alkylene oxide addition products thereof as suspension aids for the solid starter compounds. If, in this case, the alkylene oxide addition products of starter compounds solid at room temperature are used, it is also possible in principle to obtain polyether polyols based exclusively on the high-melting starter compounds in solvent-free form. Such processes are described, for example, in FR-A 1285708 and U.S. Pat. No. 3,190,927. However, the end products frequently exhibit inadequate dissolution capacity for starter compounds solid at room temperature, and in this case too, as in the case of use of solvents, a corresponding reactor volume for the suspension medium is required.
If water is used as a suspension medium/solvent for the starter compounds solid under reaction conditions, the alkylene oxide addition reaction can be stopped at a suitable point and the excess water can be removed by distillation. Such procedures are described, for example, in DE-A 1443022 and U.S. Pat. No. 4,430,490, but give rise to end products having lower functionalities as a result of the partial reaction of the water used as a suspension medium and solvent in the alkylene oxide addition. In addition, the controllability of the reaction in these so-called water methods is less good than reactions using other co-starters. It is necessary either to clean the glycol-containing wastewater formed, or to adjust the glycol content thereof to a constant value on recycling into the process. Equally, the intermediate distillative removal of the unreacted water means additional expenditure of time and energy.
It would therefore be desirable to perform the alkylene oxide addition reaction with minimum presence of solvents unreactive towards alkylene oxides and of other suspension aids at the reaction temperatures typically employed for base-catalyzed alkylene oxide addition reactions of 70 to 180° C. However, in the preparation of blockwise polyether polyols having pure ethylene oxide blocks or those having a proportion of oxyethylene units of greater than or equal to 75% by weight, it is difficult, in the absence of solvents, to obtain clear and homogeneous end products from solid starter compounds.
Prior systems fail to offer a satisfactory solution to the problems outlined with starters solid at room temperature, especially in the case of blockwise polyether polyols having a total content of oxyethylene units between 5 and 85% by weight based on the mass of all oxyalkylene units.
It was therefore an object of the present invention to provide a process for preparing polyether polyols based on starter compounds solid at room temperature, the resulting polyether polyols being formed from blocks having a content of oxyethylene units of greater than or equal to 75% by weight. The use of solvents and any great amounts of co-starters liquid at room temperature should be avoided here, and the result should be that no turbidity or inhomogeneity occurs in the end products.